This invention relates to refrigeration apparatus of the type in which an expansible compressed fluid is subjected to a thermodynamic cycle, and more specifically to reciprocating regenerative refrigerators in which the reciprocating member is driven by the fluid and both the reciprocating motion and the thermodynamic cycle are controlled by a self-regulating fluid operated valve system.
Valved reciprocating refrigerators are well known. Typical of these is the Gifford-McMahon refrigerator, in which a pair of variable volume chambers, defined by opposite faces of a reciprocating displacer and the enclosing cylinder within which the displacer slidably fits, are connected together through a regenerator. Reciprocation of the displacer forces the fluid back and forth through the regenerator. One of the two chambers is connected, through appropriate valving, to a source of the pressurized fluid and to an exhaust. Synchronization of these valves with the reciprocation of the displacer allows the apparatus to be filled with a charge of compressed fluid, which is then forced in one direction through the regenerator, next allowed to expand, and finally forced back through the regenerator. The energy loss of the fluid due to its expansion is available to cool a thermal load, the expanded fluid being forced through the regenerator, where it takes up energy, at the end of the exhaust phase of each cycle. The regenerator in turn cools the next charge of compressed fluid before the latter is allowed to expand. In steady state operation, a large temperature differential may be developed across the regenerator, and as a consequence such refrigeration apparatus may be used to develop very low temperatures while both the supply of compressed fluid and the exhausted expanded fluid are near room temperature.
The expanding fluid works against itself, not a piston (for this reason, such apparatus are often referred to as "no work" machines), the purpose of the displacer being merely to move the fluid back and forth through the regenerator once each cycle. An external source of power is required to move the displacer in close synchronization with the opening and closing of the supply and exhaust valves. While both displacer and valving may be operated electromechanically, a particularly convenient system incorporates a driving mechanism for the displacer in the form of a small piston engine driven by the fluid, and mechanically coupled and fluid actuated valving. Such apparatus is disclosed in U.S. Pat. No. 3,188,821.
Optimum heat exchange in the regenerator requires the fluid velocity in the regenerator to be constant. This requires constant velocity displacement of the displacer during the constant pressure phases of the cycle and constant pressure rise (or fall) rate during the constant displacement phases, with strict synchronization between the phases. While prior art fluid driven displacers evidence the requisite constant displacement velocity, it has proved more difficult to achieve the desired constant rate of pressure change and also the desired exact synchronization, particularly with the mechanically least complex valve systems.
An additional problem which the simplest fluid operated self-regulating valve systems encounter arises from the nature of the valve mechanism, which typically includes a pressure centered pilot spool valve. In the absence of a fluid flow (as for instance, when the refrigerator is shut down), the position of the valve is indeterminate, and automatic starting of the refrigerator is therefore not assured simply by connecting the refrigerator to a source of pressurized fluid.
Yet another problem associated with prior art fluid driven valve systems is caused by leakage of the compressed fluid between the refrigerator chambers and the piston drive chamber. Such leakage results in a less than optimum operating cycle, and has generally been solved in the prior art by the use of seals and close mechanical tolerances, with a resulting increase in friction and a larger required driving force, as well as increased cost.